The present invention relates to active matrix substrates (AM substrates), in which an active element is formed on every display pixel, used as substrates for liquid crystal displays (LCDs) and liquid crystal displays comprising the same.
Liquid crystal displays (LCDs) are becoming larger in area with higher definition. Active elements formed on every display pixel control the potentials of transparent display electrodes in active matrix LCDs (AMLCDs), thus displaying the required images.
At present, AMLCDs generally comprise transparent display electrodes made of a solid solution of indium tin oxides (ITOs) and scanning line and signal line for sending signals to the transparent display electrodes formed in one plane. With increasing definition of AMLCD displays, the proportion of scanning line and signal line and active elements taking up the display screen has increased, resulting in reduction of the effective display area. Moreover, typical AMLCDs require a certain distance to be maintained between the scanning line and signal line and transparent display electrode to prevent short-circuiting. Accordingly, a higher definition results in reduced effective display area, thus decreasing the display luminance of AMLCDs.
One of the solutions to the above problem offers an AMLCD which disposes a transparent insulating layer on the scanning line and signal line and active elements after they are formed, and then a transparent display electrode on the transparent insulating layer (improved AMLCD). A conventional improved AMLCD is described next with reference to a drawing.
FIG. 1 is a sectional view of the improved AMLCD using an AM substrate. In FIG. 1, a thin film transistor (TFT) is used as an active element of the AM substrate. Also in FIG. 1, a gate electrode 2 which also acts as a scanning line, gate insulating layer 3, semiconductor layer 4, source electrode 5 which also acts as a source line, and drain electrode 6 are formed in this sequence on substrate 1 made of an insulating substrate such as glass. The gate insulating layer is generally made of silicon nitride (SiNx).
In the manufacturing of the AMLCD, the TFT, gate line, and source line corresponding to the number of pixels are first formed in a matrix on the substrate 1. Then, transparent insulating layer 7 is formed to cover these elements. Transparent display electrode 8 is then formed on the transparent insulating layer 7, and a contact hole is created on the transparent insulating layer 7 to directly connect the drain electrode 6 and transparent display electrode 8. In FIG. 1, opposing electrode 9, black matrix 10, color filter 11, and aligning layer 12 are formed on a glass substrate opposing the AM substrate, and liquid crystal 13 is filled between the AM substrate and opposing glass substrate.
As described above, the source electrode 5 and transparent display electrode 8 are formed on different planes in the improved AMLCD. In addition, the transparent insulating layer 7 is provided between the source electrode 5 and transparent display electrode 8 so that the source electrode 5 and transparent display electrode 8 do not short circuit even if they are close in a planar distance. Moreover, the source electrode 5 and transparent display electrode 8 may also be overlaid. This makes it possible to enlarge the area of the transparent display electrode 8, thus preventing reduction in display luminance while increasing the definition of the LCD. Furthermore, the covering of the source electrode 5 with the transparent display electrode 8 makes it possible to eliminate the influence of the electric field of the source electrode 5 on the liquid crystal display. These improvements have achieved significantly higher display performance of the improved AMLCD.
However, the conventional improved AMLCD uses materials with a significantly different refractive index for each layer as shown in Table 1. For example, the liquid crystal 13 is made of organic material, the transparent display electrode 8 is made of ITO layer, the transparent insulating layer 7 is made of resin, and the gate insulating layer 3 is made of SiNx. The difference in refractive index generates interfacial reflections between layers, reducing light transmittance and resulting in a darker display.
The refractive index of the above ITO and SiNx layers may be modified slightly by adjusting the layer forming conditions. However, the refractive index of these materials is not changeable to the extent of achieving a refractive index similar to those of the liquid crystal material 13 which is organic material and the transparent insulating layer 7 which is resin material.
In addition, the transparent insulating layer 7 made of the above resin material generally has a thickness of about 2 to 3 xcexcm for two purposes: to ensure a flat surface, and to prevent electrical interference between the transparent display electrode 8 and scanning line and signal line. However, the thickness of the transparent insulating layer 7 is generally manufactured with about xc2x110% variations. This variation in the thickness generates an optical path difference of the interfacial reflective light, creating interference fringes. The occurrence of such interference fringes has an extremely deleterious effect on the LCD display quality.
Since it is difficult, under the present manufacturing conditions, to reduce variations in the thickness of the transparent insulating layer, it is necessary to suppress the occurrence of interference fringes on the precondition that the present variations in the layer thickness occur, in order to improve the LCD display quality. The present invention sets the optical characteristics of the materials comprising each layer, in particular the refractive index, to a predetermined range as one way of suppressing the interference fringes generated in the above conventional AMLCD.
The present invention provides an active matrix substrate and a liquid crystal display comprising the same having the following characteristics:
a) a transparent insulating layer is formed on an insulating substrate where an active element and scanning line and signal line are disposed in a matrix, for covering the active element and address wiring;
b) a transparent display electrode is formed on the transparent insulating layer;
c) the transparent display electrode is connected to the active element via a contact hole created on the transparent insulating layer; and
d) a difference in refractive index between an element insulating layer under the transparent display electrode which forms a part of the active element and the transparent insulating layer, and a difference in refractive index between the element insulating layer and insulating substrate are within 0.2.